This invention pertains to X-ray apparatus and is more particularly concerned with X-ray apparatus having high voltage control means.
A conventional X-ray tube has an anode target and a heated cathode. When high voltage is applied across the anode and cathode, electrons are emitted from the cathode and strike the anode with sufficient energy for the anode to emit X-rays. The energy of X-rays is dependent upon both the magnitude of the high voltage and the temperature of the cathode.
The high voltage is often supplied by a high voltage transformer while the cathode is heated by current supplied by a low voltage transformer. In some X-ray apparatus, the transformers are mounted together with the X-ray tube within a common housing. To reduce size and costs, it is sometimes the practice to wind both the high voltage windings and the low voltage windings on the same core. Both sets of secondary windings are coupled to a common primary winding which is energized by line voltage. This arrangement prevents the insulation problems known to occur when separate high and low voltage transformers are used, and allows the dimensions of the apparatus to be kept small, which is a desirable feature for dental practice.
With this arrangement, the tube is immediately under high voltage when the line supply is switched on, however, the emission of image forming radiation from the tube is retarded because of thermal inertia of the cathode. Consequently, radiation is emitted before the optimum radiation intensity required for the operation of the unit is built up. The undesirable result of this is an increase in the exposure to soft radiation which is not effective enough to form an image on X-ray film.
Because of the delay between the application of high voltage and the optimum image forming radiation, some known units include means for counting exposure time only after a certain radiation level is reached. While this method does yield reproducible results in the exposure of X-ray film, there remains the undesirable irradiation of the patient during the interval from which the high voltage is first applied to the tube until the start of exposure time. This interval will be called the cathode preheat time.
Accordingly, our object of this invention is to provide an X-ray apparatus eliminating the effects of radiation on both film exposure and the patient during the preheat time.